user
Explain why Pluto is no longer considered a planet.

assistant
Pluto is no longer considered a planet because in 2006, the International Astronomical Union (IAU) adopted a resolution that defined a planet as an object in the Solar System that is massive enough to be rounded by its own gravity and has cleared its neighborhood of smaller objects. Since Pluto did not meet this criteria, it was downgraded from a planet to a dwarf planet.